We employed a temporal two-alternative forced choice paradigm with method of constant stimuli—a classic psychophysical method [1]}, [2]}—to evaluate people's perception of surface contact. Two-alternative forced choice, or 2AFC, is a method for assessing someone's sensitivity to a change in stimuli. It involves a forced choice, because participants must chose between one of two options. A temporal 2AFC paradigm may also be referred to as a two-interval forced choice (2IFC) paradigm. After viewing both stimuli in a sequence, participants in our experiment are then asked "Which object is closer to the ground?". Compared targets were always of the same shape and shadow condition for each trial.
